Shifts In Community Structure Research Articles

Using Global-Scale Earth System Models for Regional Fisheries Applications

Climate change may impact ocean ecosystems through a number of mechanisms, including shifts in primary productivity or plankton community structure, ocean acidification, and deoxygenation. These processes can be simulated with global Earth system models (ESMs), which are increasingly being used in the context of fisheries management and other living marine resource (LMR) applications. However, projections of LMR-relevant metrics such as net primary production can vary widely between ESMs, even under identical climate scenarios. Therefore, the use of ESM should be accompanied by an understanding of the structural differences in the biogeochemical sub-models within ESMs that may give rise to these differences. This review article provides a brief overview of some of the most prominent differences among the most recent generation of ESM and how they are relevant to LMR application.

Changes in soil bacterial and fungal community composition and functional groups during the succession of boreal forests

Clarifying the response of soil microbial communities and their potential functions during succession is of great significance for understanding the biogeochemical processes and the sustainability of forest development. However, the study of microbial community dynamics during the process of succession remains poorly understood in boreal forest ecosystems. Thus, in order to study the dynamics of microbial diversity caused by succession, we adopted the "space instead of time" method and selected four habitats in a national natural reserve (including grassland, birch forest, mixed forest and larch forest) to represent the primary successional sequence of the boreal forest. We used 16S and ITS rRNA gene sequencing to detect bacterial and fungal communities and used FAPROTAX and FUNGuild database to predict bacterial and fungal functional groups. The results showed that forest succession significantly changed the community composition of bacteria and fungi, among which the fungal community was more sensitive to the changes with successional stage. The relative abundance of ectomycorrhizal fungi significantly increased with succession, while the relative abundance of bacterial functional groups involved in the nitrogen cycle did not change significantly, indicating that fungi might play a major regulatory role in the nutrient cycling process during the successional process. In this study, the soil total carbon and total nitrogen were the dominating factors affecting the soil microbial community and the structure of fungal functional groups. Our results suggest that the shifts in fungal community structure and functional groups may play a key role in soil nutrient cycling during boreal ecosystems succession.

Microbial Community Structure and Diversity in Drinking Water Supply, Distribution Systems as well as Household Point of Use Sites in Addis Ababa City, Ethiopia.

Understanding ecology of microbiomes in drinking water distribution systems is the most important notion in delivering safe drinking water. Drinking water distribution systems harbor various microbiota despite efforts made in improving water infrastructures in the water industry, especially, in developing countries. Intermittent water supply, long time of water storage, low water pressure, and contaminated source water are among many of the factors responsible for poor drinking water quality affecting health of people. The aim of this study was to explore microbial diversity and structure in water samples collected from source water, treated water, reservoirs, and household points of use locations (taps). High-throughput Illumina sequencing technology was employed by targeting the V4 region of the 16S rRNA gene and the V1-V3 region of the 18S rRNA gene to analyze the microbial community structure. Proteobacteria followed by Firmicutes, Bacteroidetes, and Actinobacteria were the core dominating taxa. Gammaproteobacteria was also dominant among other proteobacterial classes across all sampling points. Opportunistic bacterial genera such as Pseudomonas, Legionella, Klebsiella, Escherichia, and Actinobacteria, as well as eukaryotic microbes like Cryptosporidium, Hartmannella, Acanthamoeba, Aspergillus, and Candida were also abundant taxa found along the distribution systems. The shift in microbial community structure from source to point of use locations was influenced by basic factors such as residual chlorine, intermittent water supply, and long-time storage at the household. The complex microbiota detected in different sampling sites in this study brings drinking water quality problem which further causes significant health problems to both human and animal health. Treatment ineffectiveness, disinfection inefficiency, poor maintenance actions, leakage of sewage, and other domestic wastes are few among many other factors responsible for degraded drinking water quality in this study putting health at high risk. Findings of this research provide important and baseline information to understand the microbial profiles of drinking water along source water and distribution systems. Moreover, knowing the microbial profile will help to design proper water quality assurance approaches.

Miscanthus x giganteus role in phytodegradation and changes in bacterial community of soil contaminated by petroleum industry

The second generation energy crop Miscanthus x giganteus (Mxg) was cultivated in pots with mixtures of clean and petroleum industry contaminated soil affected by petroleum, Pb, Zn contamination and high salinity. The survival rate reached 100%, nevertheless the biomass parameters were negatively affected even in the lowest proportion of contaminated soil. In the lowest contamination, where the plant grew still quite successfully, C10-C40 degradation was significantly enhanced compared to the unplanted control with degradation of 58 ± 14%. The plant contribution to aliphatics degradation was significantly correlated with biomass, thus it was negligible in higher contamination. A similar pattern was documented in development of the soil bacterial community. The shift in community structure after Mxg cultivation was observed mainly in the soil with the lowest contaminant proportion, though an increase of bacterial diversity in the miscanthus rhizosphere was observed in all cases. Relative abundance of Actinobacteria was reduced on behalf of several less abundant phyla (Verrucomicrobia, Bacterioides, Acidobacteria). The majority of genera identified as potential petroleum degraders (Pseudomonas, Shinella, Altererythrobacter, Azospirillum, Mesorhizobium, Dyella) were more abundant in contaminated soil with miscanthus, suggesting that Mxg could be a promising crop for phytomanagement of petroleum contaminated soils but salt phytotoxicity needs to be mitigated first.

Assessing Spatial and Temporal Trends in a Mediterranean Fish Assemblage Structure

Coastal fishes are not only valuable elements of marine biodiversity, but they also play an important ecological role in the functioning of coastal ecosystems: food resource, transfer of nutrients, predators. Therefore, data on the compositions of fish assemblages are of great importance. The objectives of the present study were to (i) define the faunistic characteristics of a typical fish community on the Mediterranean coast; (ii) investigate spatiotemporal changes in fish assemblages. Based on a set of indices (Fast protocol) and a long-term data set (6 years, 612 visual counts, 154 h of diving), changes in coastal fish communities were analyzed. Our results indicated that there was a significant shift in fish community structure, with a general decline of the calculated indices. In our study, part of the observed variability in fish assemblage structure could be due to different factors as site location and sampled year. The changes in the fish assemblages associated with inter-annual fluctuations observed in this study also provide important insights into how fish communities may change under environmental and anthropogenic influences.

Shifts in Rhizosphere Bacterial Community Structure, Co-occurrence Network, and Function of Miscanthus Following Cadmium Exposure

As a representative of second-generation bioenergy plants, Miscanthus has received increasing attention in the studies of heavy metal (HM)-contaminated soil remediation. Currently, few studies have examined the effects of using Miscanthus to remediate HM-contaminated soils on the composition and function of microbial communities. In this study, the Miscanthus cultivar M. saccariflorus was examined for its tolerance and enrichment abilities when grown in soils containing 100 mg ·kg-1 of cadmium (Cd). The structure, function, and co-occurrence network of their rhizosphere bacterial communities were analyzed during the remediation process. MiSeq sequencing showed that the Miscanthus rhizosphere bacterial community comprised 32 phyla and 425 genera, including plant growth-promoting rhizobacteria (PGPR), such as Sphingomonas, Bacillus, Gemmatimonas, and Streptomyces. The addition of Cd affected the Miscanthus rhizosphere bacterial community and reduced community diversity. Phylogenetic molecular ecological networks indicated that Cd addition reduced the interactions between Miscanthus rhizosphere bacteria to generate a simpler network structure, increased the number of negative-correlation links, enhanced the competition between rhizosphere bacterial species, and changed the composition of key bacteria. PICRUSt functional predictive analysis indicated that Cd stress reduced soil bacterial functions in the Miscanthus rhizosphere. The results of this study provide a reference for the subsequent regulation of efficient Miscanthus remediation by PGPRs or key bacteria.

Efficient degradation of refractory pollutant in a microbial fuel cell with novel hybrid photocatalytic air-cathode: Intimate coupling of microbial and photocatalytic processes

A microbial fuel cell-photocatalysis system with a novel photocatalytic air-cathode (MFC-PhotoCat) was proposed for synergistic degradation of 2,4,6-trichlorophenol (TCP) with simultaneous electricity generation. Stable electricity generation of 350 mV was achieved during 130 days of operation. Besides, 50 mg L−1 TCP was completely degraded within 72 h, and the rate constant of 0.050 h−1 was 1.8-fold higher than MFC with air-cathode without N-TiO2 photocatalyst. Degradation pathway was proposed based on the intermediates detected and density functional theory (DFT) calculation, with two open-chain intermediates (2-chloro-4-keto-2-hexenedioic acid and hexanoic acid) detected. Furthermore, hierarchical cluster and PCoA revealed significant shifts of microbial community structures, with enriched exoelectrogen (55.2% of Geobacter) and TCP-degrading microbe (7.1% of Thauera) on the cathode biofilm as well as 61.8% of Pseudomonas in the culture solution. This study provides a promising strategy for synergic degradation of recalcitrant contaminants by intimate-coupling of MFC and photocatalysis.

Denitrification and Anammox Shift Nutrient Stoichiometry and the Phytoplankton Community Structure in the Benguela Upwelling System

AbstractA shift in nutrient stoichiometry and phytoplankton community structure due to denitrification and anammox is investigated using coupled physical‐biogeochemical model experiments in the northern Benguela. Denitrification depletes more dissolved inorganic nitrogen (DIN) than anammox, increasing phosphate accumulation and this creates a shift in nutrient stoichiometry. This decreases the nutrient stoichiometry to below the Redfield ratio with an effect on phytoplankton composition in the system. Flagellates dominate the community structure in the northern Benguela as compared to diatoms, picophytoplankton and large phytoplankton. Denitrification limits the growth of picophytoplankton resulting in a flagellate‐dominated community structure and diatom succession at depth. The phytoplankton community structure shift and succession correspond with a negative correlation between the phytoplankton groups, denitrification, anammox and DIN. The nutrient stoichiometry and a decline in phytoplankton concentrations from denitrification decrease Dissolved inorganic carbon (DIC) uptake more than gross primary production. Annual mean diatoms decline from denitrification is 0.012 Gg N , reducing DIC uptake by 2.793 Gg C and resulting in an estimated gross primary production of 0.080 Gg N in the northern Benguela. Anammox drives an estimated flagellates decline of 0.008 Gg N , a diatoms increase of 0.873 Gg N and DIC uptake of up to 46.08 Gg C . The direct links between denitrification, anammox, DIC uptake and primary production as demonstrated in this study present considerable implications for the biological pump in the northern Benguela.

Case study on a coniferous plantation site about inter-annual shifts in microbial communities under short-term detritus input manipulations

Soil microorganisms are key regulators of soil carbon (C) and nutrients cycles in terrestrial ecosystems. However, it remains uncertain how the inter-annual variation in soil microbial community corresponds well with resource availability in the changing environment. Here, we investigated soil microbial community structure and abundance, as well as the associated environmental variables from 2015 to 2017 under different plant detritus input manipulation treatments in a coniferous (Platycladus orientalis (Linn.) Franco) plantation forest ecosystem in subtropical China. Our results showed that the inter-annual variation in soil microbial community was more visible than that caused by detritus input manipulations, owing to the temporal alterations in microclimates and substrate availability. Both aboveground litter removal and root exclusion had more negative effects on the bacterial PLFAs than fungi except half a year after detritus input manipulations. While, soil microbial abundance increased only after three years of litter addition compared to control. Litter removal, especially the no input treatment significantly increased the fungi to bacteria (F:B) and Gram-positive to Gram-negative bacteria (GP:GN) ratios after one and two years of detritus input manipulations. Whereas, the litter addition treatment had minor effects on these parameters. A clear discrimination of microbial community structure among the different detritus input manipulations appeared after two and three years. Both the F:B and GP:GN ratios were positively related to the carbon to nitrogen (C:N) ratio, recalcitrance index of carbon (RIC) and nitrogen (RIN). Overall, our results reveal that the inter-annual variations in soil microbial community are clearly differentiated by the environmental variables and substrate availability that occur in different years and detritus input manipulations. Our results also suggest that due to the vital role of microorganisms in biogeochemical cycling, shifts in the microbial community structure with altered plant detritus input could profoundly affect ecosystem processes in the long run.

Long-term stability of soil bacterial and fungal community structures revealed in their abundant and rare fractions.

Despite the importance of soil microorganisms for ecosystem services, long‐term surveys of their communities are largely missing. Using metabarcoding, we assessed temporal dynamics of soil bacterial and fungal communities in three land‐use types, i.e., arable land, permanent grassland, and forest, over five years. Soil microbial communities remained relatively stable and differences over time were smaller than those among sites. Temporal variability was highest in arable soils. Indications for consistent shifts in community structure over five years were only detected at one site for bacteria and at two sites for fungi, which provided further support for long‐term stability of soil microbial communities. A sliding window analysis was applied to assess the effect of OTU abundance on community structures. Partial communities with decreasing OTU abundances revealed a gradually decreasing structural similarity with entire communities. This contrasted with the steep decline of OTU abundances, as subsets of rare OTUs (<0.01%) revealed correlations of up to 0.97 and 0.81 with the entire bacterial and fungal communities. Finally, 23.4% of bacterial and 19.8% of fungal OTUs were identified as scarce, i.e., neither belonging to site‐cores nor correlating to environmental factors, while 67.3% of bacterial and 64.9% of fungal OTUs were identified as rare but not scarce. Our results demonstrate high stability of soil microbial communities in their abundant and rare fractions over five years. This provides a step towards defining site‐specific normal operating ranges of soil microbial communities, which is a prerequisite for detecting community shifts that may occur due to changing environmental conditions or anthropogenic activities.

Responses of cucumber (Cucumis sativus L.) rhizosphere microbial community to some agronomic management practices.

The microbial communities associated to the rhizosphere (the rhizomicrobiome) have a substantial impact on plant growth and yield. Understanding the effects of agricultural management on the rhizomicrobiome is very important for selecting efficient practices. By sequencing the V4 region of 16S rRNA for bacteria and the ITS1 regions and fungi, we investigated the influences of agronomic practices, including cucumber grafting on cucurbit hybrid (Cucurbita moschata × C. maxima), cucumber-garlic intercropping, and treatment with fungicide iprodione-carbendazim on cucumber rhizosphere microbial communities during plant growth. Soil dehydrogenase activity (DHA) and plant vegetative parameters were assessed as an indicator of overall soil microbial activity. We found that both treatments and growth stage induced significant shifts in microbial community structure. Grafting had the highest number of differentially abundant OTUs compared to control samples, followed by intercropping and fungicide treatment, while plant development stage affected both alpha and beta diversities indices and composition of the rhizomicrobiome. DHA was more dependent on plant growth stages than on treatments. Among the assessed factors, grafting and plant developmental stage resulted in the greatest changes in the microbial community composition. Grafting also increased the plant growth parameters, suggesting that this method should be further investigated in vegetable production systems.

Resistance of microbial community and its functional sensitivity in the rhizosphere hotspots to drought

Climate change impacts soil microbial communities, activities and functionality. Nonetheless, responses of the microbiome in soil microenvironments with contrasting substrate availability in the rhizosphere to climatic stresses such as drought are largely unknown. To fill this knowledge gap, we coupled soil zymography with site-specific micro-sampling of the soil and subsequent high-throughput sequencing. This helped identify how the bacterial community structure and the genes encoding N-cycling enzymes (leucine aminopeptidase and chitinase) in rhizosphere hotspots and coldspots (microsites with activities in the range of bulk soil but localized within the rhizosphere) of maize respond to drought (20% WHC, two weeks).The elevated activities of leucine aminopeptidase and chitinase in rhizosphere hotspots were caused by the tight collaborative relationships between bacteria and their stable network structure rather than by any significant shift in bacterial community structure or enzyme-encoding gene copies. Despite the similarity in bacterial community structure in soil under drought and optimal moisture, functional predictions indicated the increased relative abundance of genera belonging to Actinobacteria capable of leucine aminopeptidase and chitinase production, especially Streptomyces, Nocardioides, Marmoricola, and Knoellia. Accordingly, the number of gene copies encoded by Actinobacteria for these two enzymes increased by 5.0–17% under drought. Among the bacteria with increased relative abundance under drought, Luedemannella played a crucial role in mediating nutrients and energy fluxes between bacteria. This was reflected in a 35–70% increase in leucine aminopeptidase and chitinase activities under drought. The resistance of enzyme activities to drought was higher in hotpots than that in coldspots. These results revealed that rhizosphere bacterial community composition remained stable, and that the number of gene copies encoded by Actinobacteria responsible for N-cycling enzymes increased under drought. The expected reduction of processes of N cycle was absent. Instead, bacteria increased N mining rate in those hotspots remaining active despite water scarcity.

Oral Vaccination against Lawsoniaintracellularis Changes the Intestinal Microbiome in Weaned Piglets.

Simple SummaryLawsoniaintracellularis is an obligately intracellular enteric bacterium that infects intestinal epithelial cells causing porcine proliferative enteropathy, which is responsible for economic losses in the swine industry worldwide. This study evaluated the effects of a commercial, oral attenuated L. intracellularis vaccination (Enterisol® Ileitis) on the intestinal microbial community of weaned piglets. Piglets were experimentally vaccinated with different doses of the L. intracellularis vaccine, and the gut microbial shifts were measured at weeks 0 and 6 post-vaccination. The data presented here demonstrated that oral vaccination against L. intracellularis shapes the gut microbiota in weaned piglets. Alpha diversity analysis revealed that vaccination led to significant changes in species evenness but no changes in species richness of the gut microbiota. Beta diversity analysis revealed that vaccination against L. intracellularis caused a shift in the microbial community structure. At the phylum level, Firmicutes and Bacteroidetes were the most abundant phyla in the intestinal microbiota of piglets regardless of treatment group and time points. At the genus level, there was a significant increase in Streptococcus and a significant decrease in Clostridium in the fecal microbiota of vaccinated pigs, regardless of the dose. Overall, this study sheds a new light on the potential role of the pig microbiota in modulating vaccine responses.Lawsoniaintracellularis, which causes porcine proliferative enteropathy (PPE), is a common swine intestinal pathogen that is prevalent in pig production sites worldwide. In this study, the alteration in the microbiome composition of weaned pigs was investigated in response to vaccination against L. intracellularis, using 16S rRNA gene sequencing. A total of 64 crossbred (Duroc × [Landrace × Yorkshire]) healthy weanling pigs weaned at 4 weeks of age were randomly assigned to four treatment groups (four pigs/pen; four pens/treatment), using a randomized complete block design for the 42-day trial. Pigs in the treatment groups were orally administered with three different doses (1 dose = 2 mL) of vaccine against L. intracellularis (Enterisol® Ileitis, Boehringer Ingelheim Vetmedica GmbH), namely the following: LAW1 (0.5 dose), LAW2 (1 dose), LAW3 (2 dose). A non-vaccinated group served as a negative control (CONT). Alpha diversity analysis revealed that vaccination led to significant changes in species evenness but not species richness of the gut microbiota. Beta diversity analysis revealed that vaccination against L. intracellularis caused a significant shift in the microbial community structure. At the genus level, there was a significant increase in Streptococcus and a significant decrease in Clostridium in the fecal microbiota of vaccinated pigs, regardless of dose.

Contribution of microbial photosynthesis to peatland carbon uptake along a latitudinal gradient

Abstract Phototrophic microbes, also known as micro‐algae, display a high abundance in many terrestrial surface soils. They contribute to atmospheric carbon dioxide fluxes through their photosynthesis, and thus regulate climate similar to plants. However, microbial photosynthesis remains overlooked in most terrestrial ecosystems. Here, we hypothesise that phototrophic microbes significantly contribute to peatland C uptake, unless environmental conditions limit their development and their photosynthetic activity. To test our hypothesis, we studied phototrophic microbial communities in five peatlands distributed along a latitudinal gradient in Europe. By means of metabarcoding, microscopy and cytometry analyses, as well as measures of photosynthesis, we investigated the diversity, absolute abundance and photosynthetic rates of the phototrophic microbial communities. We identified 351 photosynthetic prokaryotic and eukaryotic operational taxonomic units (OTUs) across the five peatlands. We found that water availability and plant composition were important determinants of the composition and the structure of phototrophic microbial communities. Despite environmental shifts in community structure and composition, we showed that microbial C fixation rates remained similar along the latitudinal gradient. Our results further revealed that phototrophic microbes accounted for approximately 10% of peatland C uptake. Synthesis. Our findings show that phototrophic microbes are extremely diverse and abundant in peatlands. While species turnover with environmental conditions, microbial photosynthesis similarly contributed to peatland C uptake at all latitudes. We estimate that phototrophic microbes take up around 75 MT CO2 per year in northern peatlands. This amount roughly equals the magnitude of projected peatland C loss due to climate warming and highlights the importance of phototrophic microbes for the peatland C cycle.

Taxon-specific temporal shifts in pollinating insects in mass-flowering crops and field margins in Ireland

In intensively cropped agricultural landscapes, the vegetation in edges and hedges (henceforth “field margins”) represents an important semi-natural habitat providing fundamental resources for insect pollinators. We surveyed the pollinating insects associated with two mass-flowering crops, apple and oilseed rape, and compared the insect fauna of the main crop with that in the field margins in the grass-dominated agricultural landscapes of Ireland. Different insect groups responded differently to the presence of the flowering crop, with honey and bumble bees more abundant in crops than margins during crop flowering, but more hover flies and butterflies in margins throughout. The composition of the insect assemblage also shifted over time due to taxon-specific changes in abundance. For example, solitary bees were most abundant early in the season, whereas hover flies peaked, and butterflies declined, in mid-summer. The temporal shift in insect community structure was associated with parallel changes in the field margin flora, and, although we found no relationship between insect abundance and abundance of field margin flowers, Bombus abundance and total insect abundance were positively correlated with floral diversity. After the crop flowering period, floral abundance and diversity was maintained via margin plants, but by late summer, floral resources declined. Our results confirm the importance of field margins for insect pollinators of entomophilous crops set within grass-dominated landscapes, even during the crop flowering period, and provide additional support for agri-environment schemes that protect and/or improve field margin biodiversity. The results also demonstrate that although shifts in insect and plant communities may be linked phenologically there may not always be simple relationships between insect and floral abundance and richness.

16S and 18S rRNA Gene Metabarcoding Provide Congruent Information on the Responses of Sediment Communities to Eutrophication

Metabarcoding analyses of bacterial and eukaryotic communities have been proposed as efficient tools for environmental impact assessment. It has been unclear, however, to which extent these analyses can provide similar or differing information on the ecological status of the environment. Here, we used 16S and 18S rRNA gene metabarcoding to compare eutrophication-induced shifts in sediment bacterial and eukaryotic community structure in relation to a range of porewater, sediment and bottom-water geochemical variables, using data obtained from six stations near a former rainbow trout farm in the Archipelago Sea (Baltic Sea). Shifts in the structure of both community types were correlated with a shared set of variables, including porewater ammonium concentrations and the sediment depth-integrated oxygen consumption rate. Distance-based redundancy analyses showed that variables typically employed in impact assessments, such as bottom water nutrient concentrations, explained less of the variance in community structure than alternative variables (e.g., porewater NH4+ inventories and sediment depth-integrated O2 consumption rates) selected due to their low collinearity (up to 40 vs. 58% of the variance explained, respectively). In monitoring surveys where analyses of both bacterial and eukaryotic communities may be impossible, either 16S or 18S rRNA gene metabarcoding can serve as reliable indicators of wider ecological impacts of eutrophication.

Soil properties and microbial community in the rhizosphere of Populus alba var. pyramidalis along a chronosequence

By maintaining soil structure and quality, soil microbial communities usually play important role in many forest ecosystem processes, including ecological succession. Understanding changes in the microbial communities of areas afforested with stands of different ages is of interest in ecology. Populus alba var. pyramidalis Bunge has been widely planted in Northwest China for ecological restoration. Rhizospheric soil samples were collected from 4-, 9-, 15-, 25- and 30-year-old plantations of P. alba to measure soil characteristics and soil microbial community diversity using Illumina MiSeq sequencing. The soil nutrition concentration and enzymatic activities decreased with depth of soil layer increased. In terms of stand age, alkaline phosphatase and dehydrogenase and the contents of nitrate N, available P and soil organic content (SOC) increased gradually. According to Illumina MiSeq sequencing results, the fungal and bacterial community structure varied with stand age, and diversity of fungi was less than bacteria. With increasing stand age, fungal community diversity indexes first increased and then decreased, peaked at 25y or 30y. RDA results suggested that soil available P and nitrate N were the most important factors governing fungal community structure, while available P contributed significantly to the variance of the bacterial community. Structural equation modelling (SEM) results indicated soil available P, nitrate N and SOC contents largely explained the shift in the microbial community structure along the cultivation chronosequence, and soil enzyme activities were related with changes in microbial community. Our results illustrated that the successional changes in microbial communities in the P. alba plantations can largely be attributed to changes in soil nutrition level along the chronosequence.

Coeval Drying Across the Central Tropical Pacific Over the Last Millennium

AbstractHydroclimatic variability in the tropical Pacific reflects large‐scale ocean‐atmosphere processes that imprint on global climate, but tropical Pacific hydroclimate variability over the last millennium remains uncertain. Here we present a multi‐proxy reconstruction from a lacustrine sediment record on Kiritimati Atoll that spans the last millennium. A lake salinity reconstruction from the δ2H values of total lipid extracts, combined with changes in sediment mineralogy and the stable isotopic composition of inorganic carbonates, indicate the presence of several distinct hydrologic regimes on Kiritimati, including a period of persistent aridity from 960–1030 CE, shorter episodes of aridity from 1030–1370 CE, and extreme aridity throughout the latter half of the last millennium, from 1370–1970 CE, that drove complete evaporation of the lake. A period of wetter, variable conditions defines recent decades. Compound‐specific lipid biomarker δ2H values indicate that shifts in microbial community structure occurred in the past, from a community dominated by photoautotrophic and chemoautotrophic organisms in more arid periods to a more metabolically diverse community in the recent period. Comparison of this record with nearby sediment records from Washington Island reveals a similar pattern of aridity through the last millennium, followed by wetter conditions in the modern period. These coherent changes in hydroclimate in the central Pacific are inconsistent with a migration of the Pacific Intertropical Convergence Zone within these latitudes. Rather, past aridity was caused by reduced El Niño‐Southern Oscillation variability, observed through the low‐pass filter of lacustrine records, or by multidecadal to multi‐century strengthening of the Pacific Walker Circulation.

Microbial community variation and its relationship with soil carbon accumulation during long-term oasis formation

The conversion of desert to farmland may change not only surface soils but also deeper soils due to massive irrigation and fertilization. However, limited information exists on the response of microorganisms throughout the soil profile to long-term crop development in desert regions. The shifts in the microbial community were investigated in relation to soil depth (0–2 m) and years of reclamation, as well as their relationship to soil organic carbon (SOC) dynamics in an irrigated arid area of northwest China. Oasis farmlands cultivated for 3, 5, 10, 20, 50, and > 100 years were compared with adjacent deserts. Total phospholipid fatty acids (PLFAs), bacterial PLFAs, and microbial biomass and activity within the whole profile increased from the beginning of farming and peaked at 20 years post-reclamation. Fungal and actinomycete PLFAs decreased in the beginning of reclamation but increased after 10 years and peaked at 20 years. Microbial community composition was dynamic within the first 10 years of reclamation and tended to become stable thereafter. During oasis formation, the carbon to nitrogen ratio of microbial biomass decreased from 9.0–10.5 to 7.2–8.0; the fungi to bacteria ratio decreased from 0.3–0.5 to 0.2–0.3, and the gram-positive to gram-negative bacteria ratio (GP:GN) increased from 0.3–0.4 to 0.4–0.6. Increases in microbial abundance and shifts in community structure were positively correlated with SOC accumulation. Redundancy analysis indicated that microbial changes were closely associated with increased soil moisture and nutrients and decreased salinity and pH. We believe the predominant factor driving microbial community shifts and soil carbon accumulation is the increased input of plant residue due to enhanced water and nitrogen availability after desert reclamation. This study emphasizes the significance of the microbial changes and the close association between increased GP:GN and SOC accumulation during oasis formation, especially in deeper soils (0.6–2.0 m).

Influence of prokaryotic microorganisms on initial soil formation along a glacier forefield on King George Island, maritime Antarctica

Compared to the 1970s, the edge of the Ecology Glacier on King George Island, maritime Antarctica, is positioned more than 500 m inwards, exposing a large area of new terrain to soil-forming processes and periglacial climate for more than 40 years. To gain information on the state of soil formation and its interplay with microbial activity, three hyperskeletic Cryosols (vegetation cover of 0–80%) deglaciated after 1979 in the foreland of the Ecology Glacier and a Cambic Cryosol (vegetation cover of 100%) distal to the lateral moraine deglaciated before 1956 were investigated by combining soil chemical and microbiological methods. In the upper part of all soils, a decrease in soil pH was observed, but only the Cambic Cryosol showed a clear direction of pedogenic and weathering processes, such as initial silicate weathering indicated by a decreasing Chemical Index of Alteration with depth. Differences in the development of these initial soils could be related to different microbial community compositions and vegetation coverage, despite the short distance among them. We observed—decreasing with depth—the highest bacterial abundances and microbial diversity at vegetated sites. Multiple clusters of abundant amplicon sequence variants were found depending on the site-specific characteristics as well as a distinct shift in the microbial community structure towards more similar communities at soil depths > 10 cm. In the foreland of the Ecology Glacier, the main soil-forming processes on a decadal timescale are acidification and accumulation of soil organic carbon and nitrogen, accompanied by changes in microbial abundances, microbial community compositions, and plant coverage, whereas quantifiable silicate weathering and the formation of pedogenic oxides occur on a centennial to a millennial timescale after deglaciation.